PSI - Issue 54

Liese Vandewalle et al. / Procedia Structural Integrity 54 (2024) 180–187 Liese Vandewalle/ Structural Integrity Procedia 00 (2019) 000 – 000

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Nomenclature A

pre-exponential factor

c L c T E a E B E d E T n L n T

concentration of lattice hydrogen concentration of trapped hydrogen

activation energy for hydrogen to leave trapping site binding energy of hydrogen to trapping site

energy barrier for hydrogen diffusion through the interstitial lattice energy barrier for hydrogen to enter a trapping site

total concentration of interstitial lattice sites total concentration of trapping sites present

R T

universal gas constant

temperature This beneficial effect relies on the trapping action of the carbides, preventing H to segregate at detrimental sites such as crack tips, dislocations and grain boundaries. Different trapping sites may be associated to carbides and are typically related to the elastic strain field surrounding the carbide, the carbide/matrix interface or inside the bulk of a carbide. Every H trapping site can be characterized by three energies, i.e. (1) the binding energy E B , which is the energy difference for H between trapping and interstitial lattice site, (2) the trapping activation energy, E T , which is the barrier to enter the trapping site, and (3) the de-trapping activation energy, E a , which is the barrier for H to leave the trapping site. This is schematically illustrated in Figure 1 . Clearly the three energies are related as E a is simply the sum of E T and E B . The capability of carbides to increase the HE resistance is related to the amount of H that can be stored in the trapping sites they provide as well as the difficulty for H to leave these traps. Hence, most studies focus on E a and the trap density (n T ). However, it must be noted that the amount of H that is trapped is not only determined by n T but also by the driving force for trapping, being determined by E B , and sometimes even by E T . Indeed, for titanium carbides different trapping sites are shown to be activated when H is introduced via electrochemical charging at room temperature compared to gaseous charging at elevated temperatures. Electrochemical charging at room temperature mainly resulted in filling of trapping sites related to the TiC/matrix interfaces and small semi-coherent precipitated carbides were shown to be most important providers of trapping sites (Depover and Verbeken (2016a), Drexler et al. (2019)). On the other hand, gaseous charging at temperatures of around 500°C and higher resulted in trapping by C-vacancies in the carbide bulk and the large incoherent carbides which were not dissolved upon austenitization were found to be the main providers of these trapping sites (Pérez Escobar et al. (2013), Vandewalle et al. (2023), and Wei and Tsuzaki (2006)). This was reflected in TDS spectra of electrochemically charged specimens that contained desorption peaks in the region from 50 °C to 450 °C while gaseous charging only resulted in the appearance of a high temperature desorption peak between 500 °C and 700 °C. This can be related to the difference in E T and E B , i.e. the bulk C-vacancies are characterized by a high E T and E B , while the trapping sites related to the semi-coherent interfaces have moderate values for E B and low values for E T (around E d ). Hence, H atoms with low thermal energy (electrochemical charging) cannot overcome the high E T barrier associated with the bulk C-vacancies but will be trapped by the semi-coherent interfaces. While H atoms with high thermal energy, as for gaseous charging, can overcome the high E T barrier of the bulk C-vacancies and are trapped there but will not be trapped in the interface related traps due to their lower E B and E a .

Figure 1: Schematic illustration of hydrogen energy profile.